This Phase I/II open-label randomized trial of pediatric patients previously exposed to and progressing on current antiretrovirals to study a combination of four drugs to primarily assess antiretroviral effect. Patients who have no prior exposure to protease inhibitors or to a non-nucleoside reverse transcriptase inhibitor (NNRTI) will be randomized to 2NRTI's + Nevirapine + Nelfinavir or to 2NRTI''s + Nevirapine + Retonivir. The NRTI's will be different from the patient's current regimen. For patient's previously exposed to protease inhibitors without prior exposure to NNRTI's, they will receive 1NRTI + Nevirapine + Nelfinavir + Retonivir. For patient's with no prior experience to protease inhibitors, but prior exposure to NNRTI as well as patients exposed to both, they will receive 2NRTI's + Nelfinavir + Retonavir. The primary objectives of the study are to determine the proportion of subjects having a reduction in HIV-1 RNA; to assess toxicity; and to determine steady-state pharmacokinetics of nevirapine, nelfinavir, and retonavir when used incombination with respect to age and treatment group. Multiple secondary objectives are also stated din this complex study.